Micromechanical structure

ABSTRACT

The invention relates to a micromechanical structures that include movable elements. In particular the invention relates to an arrangement for coupling such movable elements to other structures of a microelectromechanical system (MEMS). The invention is characterized in that the arrangement comprises at least one coupling means ( 930 - 936 ) for coupling the movable element to the fixed structure, and at least one flexible means ( 980, 990 - 996 ) for allowing different thermal expansion between the movable element and the other structure in the direction which is substantially perpendicular to the characteristic movement of the movable element, wherein said coupling means and/or flexible means is reinforced in the direction of the characteristic movement of the movable element.

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a micromechanical structures thatinclude movable elements. In particular the invention relates to anarrangement for coupling such movable elements to other structures of amicroelectromechanical system (MEMS).

BACKGROUND ART OF THE INVENTION

[0002] In microelectronics the trend has been towards a higher level ofintegration. The same applies to micromechanics as well. Consequently,micromechanical elements designated especially for microelectronicpurposes need to be more highly integrated because of the requirementfor smaller and smaller components for electrical applications.

[0003] Prior art micromechanical components have been optimized for lowfrequency (<1 MHz) applications and used mainly for inertial andpressure sensors. The design of micromechanical RF components for 1 to 5GHz applications used in mobile terminals sets demands on micromachinedstructures. These demands are partly different from the problems in thelow frequency Micro Electromechanical Systems (MEMS) applications.

[0004] The optimization of the capacitive micromechanical structures issubject to several parameters:

[0005] Sensitivity to the measured value or control force (e.g.,acceleration to capacitance transfer function, control voltage tocapacitance transfer function),

[0006] Signal to noise ratio that depends on the several other deviceparameters,

[0007] Zero point stability of the device with respect to long timeperiods and temperature.

[0008] These optimization criteria convert into more specific devicerequirements when the application and especially the measurement oroperation frequency is taken into account. This invention is related tothe use of the micromechanical structure as a part of the high frequencyapplication. Two different examples of such an application are:

[0009] MEMS rf components: tunable capacitors and micromechanicalmicrorelays;

[0010] Micromechanical low noise, high sensitivity accelerometer usingLC resonance as a basis of the measurement electronics;

[0011] For both these applications, there are several commonrequirements for the device:

[0012] Series resistance of the device must be minimized;

[0013] Series (stray) inductance of the device must minimized andrepeatable;

[0014] Temperature dependence of the structure must be as small aspossible; and

[0015] Parasitic capacitance must be minimized.

[0016] The prior art micromechanical structures are mostly based onsilicon and polysilicon structures. The polysilicon has good mechanicalproperties and technology to build suspended structures from it is wellresearched. However, the main disadvantage of these structures is thehigh series resistance. The series resistance reduces the Q value of thecomponent at high frequencies.

[0017] Many devices like the low-noise rf voltage controlled oscillators(VCO) require a resonant device with high Q-factor, since the phasenoise of an oscillator is proportional to 1/Q_(T) ², where Q_(T) is theoverall Q-factor of the resonator. High dynamic range filters alsorequire a high Q-resonator, since the dynamic range of the filter isproportional to Q_(T) ². The quality factor within the frequency range 1to 2 GHz is dominated by the series resistance. Previously, for instancethe MEMS tunable capacitors were fabricated from the polysilicon, butthe requirement for the low series resistance has forced to considermetal as the material of the structure. Metal can be for instance gold,copper, silver, nickel, aluminium, chromium, refractory metal or alloyof several metals.

[0018] In capacitive sensors the ultimate resolution of the capacitancemeasurement is limited by the series and/or parallel resistances of thesensing capacitance. Most of the prior art capacitive inertial sensorsare made of doped monocrystalline or polycrystalline silicon, and theconductivity is limitted to relatively modest values. Furthermore, theadditional series resistance due to metal/silicon interfaces increasesthe series resistance. The inertial sensors based on metal structureshave been studied, [1] and [2], because of two clear advantages: 1)metals have higher material density that increases the mass and thus thesensitivity of the capacitive sensor, and 2) metals have higherelectrical conductivity that reduces the electrical noise of thecapacitive sensor. One of the key problem in using metallic materialsfor inertial sensors has been the elimination of thermal stress causedby the mismatch of the thermal expansion coefficients between thesubstrate and the structure.

[0019] Metal thus has some disadvantageous characteristics like thebuilt-in stress that can cause warping of the suspended structures. Inaddition, most metals that are available in the MEMS processes have thethermal expansion coefficient that is very different compared to thethermal expansion coefficient of most substrate materials such assilicon, quartz or borosilicate glass. Thermal stress of the suspendedstructure, due to the thermal expansion mismatch, can cause severethermal dependence in the device.

[0020]FIG. 1 shows a typical micromechanical bridge. The requirement isto make a mechanically ideal anchor using a minimum of process steps. Asimple process is advantageous in the method shown in FIG. 1. Onedisadvantage of such metal structure is that the built-in stress and anytemperature dependent stress tend to bend the suspended structure.

[0021]FIG. 1 illustrates the situation when the micromechanical metalstructure with a movable element 110 and anchors 130, 132 is depositedon top of the silicon substrate 150. The FIG. 1 also shows theinsulating layer 160 and the fixed electrodes 140, 142 on the substrate.The change of the internal stress of the metal 110 due to the change intemperature can be calculated as

Δσ=E·(α₁−α₂)·ΔT  (1)

[0022] where E is the Young's modulus, α₁ and α₂ are the thermalexpansion coefficients of the metal film and the silicon substrate,respectively, and ΔT is the temperature change.

[0023] For the copper film on top of the silicon substrate,$\begin{matrix}{\frac{\partial\sigma}{\partial T} = {2{\frac{MPa}{{^\circ}\quad C}.}}} & (2)\end{matrix}$

[0024] The stress in the metal causes a force F_(eff) to the anchoringstructures 130 and 132.

[0025]FIG. 2 shows the moment effect at the step-up anchor structure. Wesuppose that the suspended structure is connected to the substrate fromseveral points and that the thermal expansion mismatch between thesubstrate and the suspended structure causes strain in the suspendedstructure. Effect of the strain is shown as two arrows in FIG. 2. Figureshows how the moment caused by the step-up anchor bends (exaggerated)the suspended structure. Normal dimensions for the suspended structuremight be for instance that the suspended structure is 500 μm long, 1 μmthick and it is 1 μm above the substrate. Even a very small bendingmoment would be catastrophic, since the structure would touch thesurface.

[0026]FIG. 3 shows how the control voltage is dependent on the residualstress of a copper film double supported beam. The capacitance is keptconstant, in this case 0.9 pF. Length of the beam is 0.5 mm, width is0.2 mm, and thickness is 0.5 μm. The gap between the control electrodeand the beam is 1 μm. The Figure shows how sensitive the control voltageis to low level residual film stress.

[0027] The temperature dependence of the capacitance can be calculatedas $\begin{matrix}{\frac{\partial C}{\partial T} = {\frac{\partial C}{\partial\sigma} \cdot \frac{\partial\sigma}{\partial T}}} & (3)\end{matrix}$

[0028] The temperature dependence increases with the control voltage.For example, for 5 MPa residual stress, the temperature dependence ofthe capacitance can be 1 %/° C. at 1 V control voltage, and 24 %/° C. at3 V control voltage. If the device is operated at low control voltages,the residual stress of the film must be minimized. At this range, thetemperature dependence must be minimized by some structuralmodifications.

[0029] The temperature dependence has been reduced by using flexiblespring support for the structure. Such prior art solutions forimplementing micromechanical components are described e.g. in documents[3]-[6]. However, the problem of these prior art devices is: 1) too highseries resistance, 2) too high temperature dependence, 3) too high strayinductance.

[0030] Prior art micromechanical structures comprising movable elementshave therefore disadvantages related to the requirements describedabove. The prior art structures suffer from temperature dependence, dueto the mismatch of thermal expansion coefficients of the micromechanicalstructure and the substrate. Series resistance and parasitic capacitanceare also high in prior art RF components such as tunable capacitors andresonators based on a tunable micromechanical capacitor and anintegrated inductor. These factors may lead to high losses, thermalunstability and unreliability of the micromechanical components.

SUMMARY OF THE INVENTION

[0031] The purpose of the invention is to achieve improvements relatedto the aforementioned disadvantages. The invented arrangements forcoupling a movable element to other micromechanical structuresfacilitates minimizing the temperature dependence, the seriesresistance, the stray inductance and the parasitic capacitance. Hence,the invention presents a substantial improvement to the stability andreliability of the micromechanical componets, especially in the RFapplications.

[0032] An arrangement according to the invention for coupling a movableelement, which has a characteristic movement direction, to a fixedstructure, such as substrate, of a micromechanical component, ischaracterized in that the arrangement comprises at least one couplingmeans for coupling the movable element to the fixed structure, and atleast one flexible means for allowing different thermal expansionbetween the movable element and the other structure in a direction whichis substantially perpendicular to the characteristic movement of themovable element, wherein said coupling means and/or flexible means isreinforced to be substantially inflexible in the direction of thecharacteristic movement of the movable element.

[0033] The invention also relates to a micromechanical component wichcomprises an arrangement described above.

[0034] Preferred embodiments of the invention are described in thedependent claims.

[0035] One idea in implementing this invention is to use an additionallayer, such as a metal layer, to form boundary conditions that are asclose to ideal as possible for suspended structures. The inventiveconcept can most advantageously be realised using one or several of thefollowing details:

[0036] 1) The deflecting metal thin film is mechanically decoupled fromthe substrate and consists of:

[0037] a) Membrane, diaphgram or thin metal film of any shape,

[0038] b) Surrounding frame that can be of any shape as long as it issymmetric about the axes formed by two opposing anchors,

[0039] c) Inner springs that connect the deflecting element to the frameare formed on the corners of the frame,

[0040] d) Anchoring of the frame to the substrate at the middle of theframe forming beams,

[0041] e) Optional outer beams that further connect the frame and thesubstrate anchoring. The structure is further characterized by thesymmetry shown in FIG. 9A (described in more detail in the followingpart of the specification), and

[0042] f) Anchoring of the frame to the substrate is arranged to betemperature compensated.

[0043] Mechanical decoupling of the movable element achieved by thestructure is almost perfect. Disadvantage of the planar structure ofthis preferred embodiment is, however, that the corners of the frame maywarp in the direction perpendicular to the substrate plane (in verticaldirection) due to the built-in (residual) stress in the frame or movingelement.

[0044] 2) Eliminating of the warping of the structure by having largervertical thickness for the frame than for the moving element. Anotherpossibility to achieve a rigid vertical structure is to use profilegeometries.

[0045] The invention can be implemented utilizing new fabricationtechnologies that are commonly known as micro system technologies (MST)or Micro Electromechanical Systems (MEMS). These fabricationtechnologies enable the fabrication of movable structures on top of thesilicon wafer or any other substrate material. The preferred process isbased on the deposition of a sacrificial material layer (silicon dioxideor polymer film) under the movable structure during the fabrication.During the final steps of fabrication the movable mechanical structureis released by etching the sacrificial layer away.

[0046] Invention improves the prior art devices (metal film structureson top of silicon substrate) in several ways:

[0047] Thermally induced stress of the deflecting thin film isminimized, below 0.5 MPa level, because of the geometrical symmetries;

[0048] Series resistance is low, below 0.1 Ω, because of eight parallelcurrent paths from the thin film to the anchor;

[0049] Series (stray) inductance is low, below 0.1 nH, because of eightparallel current paths from the thin film to the anchor;

[0050] Low control voltage level possible (3-5 V) because of the lowfilm stress; and

[0051] Warping of the mechanically decoupled structure is small.

[0052] Removes almost all the stress issued to the suspended structuredue to the thermal expansion mismatch.

[0053] Relaxes the built-in stress in the suspended structure.

[0054] Series resistance of the spring structure is smaller than in theprevious spring structures.

[0055] Very rigid structure in other degrees of freedom. Rigidboundaries prevent warping and allow bigger capacitors to be made, thanprevious structures.

[0056] Eliminates the moment effect caused by thermal deformation of thethick anchoring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Next the invention will be described in greater detail withreference to exemplary embodiments in accordance with the accompanyingdrawings, in which

[0058]FIG. 1 illustrates a prior art micromechanical bridge,

[0059]FIG. 2 illustrates a moment effect in an anchor of a prior artmicromechanical bridge,

[0060]FIG. 3 illustrates dependence of pull-in voltage as a function ofresidual stress of a bridge in a prior art micro-electromechanicalcapacitor,

[0061]FIG. 4A illustrates an example of an anchor according to theinvention,

[0062]FIG. 4B illustrates deformation of a thick anchoring,

[0063]FIG. 4C illustrates an example of symmetric anchoring according tothe invention that eliminates the effect caused by the deformation ofthick anchoring,

[0064]FIG. 4D illustrates another example of symmetric anchoringaccording to the invention that eliminates the effect caused by thedeformation of thick anchoring,

[0065]FIG. 5 illustrates an example of a micromechanical bridgeaccording to the invention,

[0066]FIG. 6 illustrates a cross section of spring and anchor elementsin an examplary michromechanical bridge according to the invention,

[0067]FIG. 7 illustrates an example of a micromechanical bridgeaccording to the invention comprising one spring element,

[0068]FIG. 8 illustrates an example of a coupling structure of a squareelectrode plate according to the invention,

[0069]FIG. 9A illustrates a preferable coupling structure of arectangular diaphragm according to the invention,

[0070]FIG. 9B illustrates a diaphragm suspended by an anchored frame,

[0071]FIG. 9C illustrates a symmetrical frame anchored by two anchors,

[0072]FIG. 9D illustrates typical dimensions of the coupling structureof FIG. 9B,

[0073]FIG. 9E illustrates a simplified top and cross-sectional view of abridge capacitor having frame beams reinforced with profile geometry,

[0074]FIG. 9F illustrates an acceleration sensor with a temperaturecompesation structure according to the invention,

[0075]FIG. 10 illustrates ac electrical equivalent circuit of amicromechanical capacitor,

[0076]FIG. 11A illustrates a first embodiment of a coupling structurewith a frame for a rectangular electrode plate, according to theinvention,

[0077]FIG. 11B illustrates a first embodiment of a coupling structurewith a frame for a rectangular electrode plate, according to theinvention,

[0078]FIG. 11C illustrates a first embodiment of a coupling structurewith a frame for a rectangular electrode plate, according to theinvention,

[0079]FIG. 11D illustrates a first embodiment of a coupling structurewith a frame for a rectangular electrode plate, according to theinvention,

[0080]FIG. 12A illustrates cross sections of a production sample afterphases 1210-1240 in an examplary process to produce a structureaccording to the invention, and

[0081]FIG. 12B illustrates cross sections of a production sample afterphases 1250-1270 in an examplary process to produce a structureaccording to the invention.

DETAILED DESCRIPTION

[0082] FIGS. 1-3 were explained above in describing the prior art.

[0083]FIG. 4A illustrates a cross section of an examplary anchoraccording to the invention. A thick second layer 430B is deposited overthe area 430A that forms the anchoring of the suspended structure 410.This second layer will eliminate distortion effects on the suspendedstructure due to bending of the step up anchor structure, and the secondlayer also reduces the series resistance of the device if it is made ofconductive material. The second layer is preferably a metal layer, butit may also be made of other material. When micromechanical anchoringstructure is thick, there is significant deformation of the anchoringstructure due to the thermal expansion of the diaphragm 410. This isillustrated in FIG. 4B. Figure shows that when the anchoring 430 isfixed to the substrate 450, its bottom cannot chance its size withtemperature. However, the upper part of the thick anchoring structuremay chance its size with temperature. This creates a momentum M to thesuspended structure that causes temperature dependence in the devicebehavior.

[0084]FIGS. 4C and 4D illustrate top views and cross section views oftwo anchoring solutions that eliminate this effect. The solutions arebased on an anchoring structure with two fixing points that aresymmetrically connected to the substrate so that the moments from bothfixing points cancel each other. In the solution of FIG. 4C there aretwo fixing points 430 p and 430 q that are placed symmetrically around asection of the frame 480 so that the moment caused by one of the firstfixing point 430 p is cancelled by the moment caused by the secondfixing point 430 q. In the solution of FIG. 4D there are also two fixingpoints 430 r and 430 s that are placed symmetrically around a projectingpart 481 in a section of the frame 480 so that the moment caused by oneof the first fixing point 430 r is cancelled by the moment caused by thesecond fixing point 430 s.

[0085]FIG. 5 illustrates an example of a micromechanical bridgeaccording to the invention. The bridge comprises a spring structure 570,572 between the suspended structure 510 and anchors 530 and 532. Thespring structure relieves the stress that is caused by the thermalexpansion mismatch between the substrate and the suspended structure. Inaddition, the spring structure releases the built-in stress that isformed to the suspended structure during the manufacturing.

[0086]FIG. 6 illustrates a cross section of spring and anchor elementsin an examplary michromechanical bridge according to the invention. Theanchor 630 consists of a first metal layer 630A and a second metal layer630B. The reinforced structure is also used in the spring element 670which thus consists of a first metal layer 670A and a second metal layer670B. FIG. 6 also shows the suspended structure 610 and the substrate650.

[0087] There are several implementations for the spring structure whenapplied in tunable capacitors. A first implementation was illustrated inFIGS. 4 and 5 wherein a spring structure is used in both ends of thebeam to lower the temperature dependency, without substantiallyincreasing the series resistance. FIG. 7 shows a second implementation,where a single reinforced spring 770 is located in the center of thebeam and divides the beam into two parts 710 and 712. The anchors 730and 732 are coupled directly to the the two parts 710, 712 of the beam.

[0088]FIG. 8 illustrates an examplary coupling structure of a squareelectrode plate according to the invention. In this embodiment thesecond metal layer forms a reinforced frame, 880, 880A, 880B, thatprovides firm boundary conditions for the movable electrode 810, thuspreventing warping of the diaphragm capacitor structure. Warping tendsto limit the size of the thin film capacitor, so the firm boundaryconditions due to the second metallisation layer allow realization ofmuch bigger structures and in addition lower the series resistance. Theframe 880 is coupled with four springs 870, 872, 874 and 876 to anchors830, 832, 834 and 836. Both the anchors and the springs have areinforced structure (872A, 872B, 876A, 876B).

[0089]FIG. 9A illustrates a preferable coupling structure using a framefor a rectangular electrode plate, in accordance with the invention. Inthis embodiment the second metal layer forms a reinforced frame, 980,which is coupled to the movable thin film 910 from corners with innersprings 990, 992, 994 and 996, thus preventing warping of the thin filmcapacitor structure. The frame is coupled to the substrate with fouranchors 930, 932, 934 and 936 that are coupled to the frame with outerbeams 970, 972, 974 and 976 that also serve as springs. The anchors andthe springs may also have a reinforced structure.

[0090]FIGS. 9B and 9C show other possible geometries for an arrangementwith a frame. FIG. 9C illustrates an embodiment of a frame 980 symmetricalong the axis 957 between two anchoring points 930 and 932, to whichthe frame is flexibly attached. A transversal beam 955 is arranged tomake the frame rigid in absence of further anchoring points. Thediaphragm 910 is attached to this rigid frame. FIG. 9D shows typicaldimensions of the sturcture that is illustrated in FIG. 9B. The typicaldimensions shown in the Figure are in micrometers. The framework aroundthe thin film that compensates the thermal stresses is typically about20 μm wide and 10 μm thick. The frame is stiff enough to prevent warpingof the structure. When the thin film in the center acts as tunablecapacitor, its typical thickness is 1 μm. A typical size of the thinfilm side is 50-500 μm. FIG. 9D also illustrates how outer beams 998,999 are used in order to decouple any length expansion of the diaphragmfrom reaching the substrate via the anchors 930 and 932.

[0091] The embodiments of FIGS. 9A-9B have the following preferablefeatures:

[0092] a) The thin film 910 is rectangular, preferably square;

[0093] b) The surrounding frame 980 is of a continuous, rectangular(square) structure;

[0094] c) Inner springs 990, 992, 994 and 996 connect the thin film tothe frame at the corners of the frame;

[0095] d) The frame 980 is anchored to the substrate at the middle ofthe frame forming beams;

[0096] e) The structure may have optional outer beams that furtherconnect the frame and the substrate anchoring.

[0097] The structure is preferably symmetrical. The anchoring of theframe to the substrate and the attachment of the thin film to the frameare preferably at 45 degree angle of each other. Mechanical decouplingof the diaphragm from the substrate achieved by the structure is at anoptimum.

[0098] Measurements show that the structure according to FIG. 9A almosttotally prevents changes of stress in the thin film due to temperaturechanges. The frame around the thin film deforms under the thermalstress, but the thin film remains mostly intact. In a case where aconventional bridge structure would have a thermal stress of 100 MPainduced in the suspended thin film, the structure of FIG. 9A wasmeasured to have a thermal stress of less than 0.5 MPa in the thin film.In this measurement the temperature change was 50 degrees C., which ispossible in the enviroment of mobile devices.

[0099] The frame can be reinforced against the movement in the directionof the characteristic movement of the thin film, as shown above, byproducing the whole frame thick in this direction by using thickermaterial in the frame. However, another way of reinforcing the frame isto use a profile geometry for the cross section of the frame. Thegeometry of the beams may may have the shape of eg. “U” “T” profile.FIG. 9E shows a simplified top view and cross section view of a bridgecapacitor that is surrounded with a frame 980 which is reinforced withprofile geometry. This reinforcement may be used not only in theembodiments using a frame such as the examples of FIGS. 9A-9D, but alsoin other embodiments such as the ones illustrated in FIGS. 5-7.

[0100]FIG. 9F shows how the acceleration sensor with inertial mass 914can be implemeted using invented temperature compensation structure.According to the theory the most accurate method to measure thedisplacement in the capacitive structure is to tune the capacitivesensor by an inductor. The improvement of the resolution of thecapacitive sensor in tuning it by an inductor is inversely proportionalto the Q value of the tuned circuit. The conclusion is that the rfmeasurement principle improves the measurement resolution only if the Qvalue of the tuning circuit is relatively high, i.e., Q>100. The stateof the art micromechanical accelerometers have large series resistanceand thus low Q value. Accelerometer according to the present inventionusing 400×400 μm² plate with thickness of 12 μm, enables anaccelerometer with 10⁻² μm/g sensitivity that is optimal for 50 gmeasurement range.

[0101]FIG. 10 illustrates an electrical equivalent circuit of thetunable capacitor that is shown in FIG. 9A. The citations as well assome typical values for the electrical parameters of the structure ofFIG. 9A are listed in Table 1. TABLE 1 Parameters in the electricalequivalent circuit of FIG. 10. Parameter Description Value C Capacitancein the air gap (1 μm thick) 1.0 pF Cd Capacitance in the dielectriclayer (100 nm 44 pF thick) Rs_1 Resistance in the lower electrode (1 μmthick) 0.05 Ω Ls_1 Inductance in the lower electrode (1 μm thick) 0.05nH Rs_2 Resistance in the upper electrode (0.5 μm thick) 0.1 Ω Ls_2Inductance in the upper electrode (0.5 μm 0.11 nH thick) Rs_3 Resistancein the frame (10 μm thick) 0.06 Ω Ls_3 Inductance in the frame (10 μmthick) 0.18 nH Rs_4 Resistance in the frame (10 μm thick) 0.03 Ω Ls_4Inductance in the frame (10 μm thick) 0.14 nH Cp_1 Parasitic capacitanceto the substrate 0.1-0.5 pF Rp_1 Resistance in the substrate ˜10 kΩ Cp_2Parasitic capacitance to the bias electrode 1 pF Rp_2 Impedance of thecontrol circuit >1 kΩ

[0102] The values in the Table 1 show that the series resistance andinductance values are very small which makes the capacitor structurevery suitable for high frequency applications.

[0103] FIGS. 11A-11D show four implementations of a tunable capacitorwith a coupling frame, and how a tunable capacitor can be connected intoa coplanar waveguide (CPW) line. In the embodiments of FIGS. 11A and 11Bthe frame 1180 is grounded to the ground lines 1140, 1142, from twoattachment points 1132, 1136, and in the embodiments of FIGS. 11B and11D the frame is grounded to the ground lines 1140, 1142, from all fourattachment points 1130, 1132, 1134, 1136. The thin film 1110 isconnected to the frame 1180 from all corners both mechanically andelectrically.

[0104] In the embodiments of FIGS. 11A and 11B the signal electrode 1145is used also as a control electrode, but in the embodiments of FIGS. 11Cand 11D comprise a separate signal electrode 1146, and the capacitanceis controlled with separate control electrodes 1147, 1148. In FIGS. 11Cand 11D the signal and control electrodes under the thin film 1110 arealso shown; the thin film itself can be similar in all four embodiments.

[0105] In the embodiments of FIGS. 11A-11D the movable thin film isgrounded, and the “hot” signal and control electrodes are fixed on thesubstrate, which is more convinient for providing the electricalconnections to the thin film. This way the parasitic capacitance betweenthe hot electrode of the capacitor and the substrate ground potentialcan als be minimized. However, it is also possible to use the movablethin film as a hot electrode, and use the fixed electrode of thesubstrate as a ground electrode.

[0106]FIGS. 12A and 12B describe phases of a typical process to make theinvented structure. Protective nitride layer 1212 is first grown firston the substrate 121 land a polymer layer 1213 is deposited on thenitride layer, step 1210. Polymer can be deposited for instance byspinning. On the next phase 1220 the first lithography is performed, andthe anchor opening is patterned on the polymer. This is followed by thestep 1230 of seed layer 1234 deposition and patterning of the seed layerand followed by the step 1240 of electroplating. The firstelectroplating creates a thin (eg. a thickness of 1 μm) metal layer 1245over the polymer sacrificial layer.

[0107] A second polymer layer 1256 is then deposited on step 1250 and athird lithography step is used to partially remove the polymer. Now partof the first metal structure is visible and it is used as a seed layerfor the second electroplating, step 1260. This electroplating forms thethick metal layer (eg. a thickness of 10 μm), 1267, which stabilizes theanchor and forms and reinforces the springs. In the last step 1278,sacrificial polymer is etched away, 1278, and the suspended structure isthus released.

[0108] The invention has been explained above with reference to theaforementioned embodiments, and several industrial advantages of theinvention have been demonstrated. It is clear that the invention is notonly restricted to these embodiments, but comprises all possibleembodiments within the spirit and scope of the inventive thought and thefollowing patent claims. For example, the inventive idea of themicromechanical arrangement is not restricted to be used in a tunablecapacitor, but it can be applied also in many other components andpurposes. One examplary application of the invention is an inertialsensor, such as an accelometer or an angular rate sensor where it ispossible, with the present invention, to achieve a low series resistanceand high Q value together with large inertial mass. The invention is noteither restricted to use of the mentioned materials. For example, thereinforced structure may comprise thin film and/or electroplated metal,it may comprise polycrystalline silicon and/or monocrystalline silicon,or it may comprise insulating films.

CITED REFERENCES

[0109] [1] Y. Konaka and M. G. Allen, “Single- and multi-layerelectroplated microaccelerometers”, Digest of Tech. Papers, IEEE 1996.

[0110] [2] J. T. Ravnkilde, “Nickel surface micromachinedaccelerometers”, Internal Report, MIC-DTU, August 1998.

[0111] [3] Dec A. and K. Suyama, Micromachined electro-mechanicallytuneable capacitors and their aplications to RF IC's, pp. 2587-2596,IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 12,1998.

[0112] [4] Gill J., L. Ngo, P. Nelson and C-J Kim, Elimination of extraspring effect at the step-up anchor of surface-micromachined structure,Journal of microelectromechanical systems, pp. 114-121, Vol. 7, No. 1,1998.

[0113] [5] Nguyen C., L Katehi and G. Rebeiz, Micromachined devices forwireless communications, pp. 1756-1768, Proc. IEEE, Vol. 86, No. 8,1998.

[0114] [6] D. J. Young, J. L. Tham, and B. E. Boser, AMicromachine-Based Low Phase-Noise GHz Voltage-Controlled Oscillator forWireless Communications, Proc. of Transducers '99, June 7-10, 1999,Sendai, Japan, pp. 1386-1389).

1. An arrangement for coupling a movable element, which has acharacteristic movement direction, to a fixed structure, such assubstrate, of a micromechanical component, characterized in that thearrangement comprises at least one coupling means for coupling themovable element to the fixed structure, and at least one flexible meansfor allowing different thermal expansion between the movable element andthe other structure in a direction which is substantially perpendicularto the characteristic movement of the movable element, wherein saidcoupling means and/or flexible means is reinforced to be substantiallyinflexible in the direction of the characteristic movement of themovable element.
 2. An arrangement according to claim 1, characterizedin that said flexible means is located between said movable element andsaid coupling means.
 3. An arrangement according to claim 1,characterized in that flexible means is located within said movableelement.
 4. An arrangement according to claim 1, characterized in thatthe flexible means comprises a frame which is connected to the movableelement and to the coupling means for connecting the movable elementflexibly to the coupling means.
 5. An arrangement according to claim 1,characterized in that the reinforcing is achieved with an enlargedthickness of the material.
 6. An arrangement according to claim 1,characterized in that the reinforcing is achieved with a profilegeometry.
 7. An arrangement according to claim 1, characterized in thatthe reinforced structure comprises thin film and/or electroplated metal.8. An arrangement according to claim 1, characterized in that thereinforced structure comprises polycrystalline silicon and/ormonocrystalline silicon.
 9. An arrangement according to claim 1,characterized in that the reinforced structure comprises insulatingfilm.
 10. An arrangement according to claim 4, characterized in that theframe is attached to the movable element from the inside of the frame,and the frame is attached to the coupling means from the outside of theframe.
 11. An arrangement according to claim 4, characterized in thatthe coupling means are connected to a fixed structure at least at twoseparate anchoring points symmetrically positioned around a part of saidframe.
 12. An arrangement according to claim 4, characterized in thatthe frame is attached to the movable element from the comers of theframe, and the frame is attached to the coupling means from the middleof the frame beams.
 13. An arrangement according to claim 1,characterized in that the movable element is a deflecting diaphragm. 14.An arrangement according to claim 1, characterized in that the movableelement is an electrode of an adjustable capacitor.
 15. An arrangementaccording to claim 1, characterized in that the movable element hasenlarged thickness forming an inertial mass of an inertial sensor.
 16. Amicromechanical component, characterized in that it includes anarrangement according to claim 1.